Large families reduce the chances of survival for parents, according to a new study by researchers at the University of Utah (the U). The study, published in the current issue of The Proceedings of the National Academy of Sciences (PNAS) and entitled “Differential fitness costs of reproduction between the sexes,” analyzed data from over 21,000 couples married between 1860 and 1895 identified in the Utah Population Database. The research team, led by Dr. Ken Smith at the University of Utah and Dr. Dustin Penn at the Konrad Lorenz Institute of Ethology, found that the more children couples had, the greater their risk of mortality.
With data collected from the 21,684 couples and their 174,000 children, researchers concluded that increased family size was associated with decreased survival for both parents, although significantly more for mothers. According to Smith, “Larger family size was also associated with lower offspring survival, especially for later-born children. In neither case did other factors such as economic status play a role in the survival rates.” He also notes that, “Our results are consistent with the idea that reproduction requires a trade-off between quality and quantity, and may help explain the evolution of menopause as a means of increasing mother survival.”
BBC News, “Large families bad for parents,” reported that “the findings do shed light on human reproduction which are still relevant today. Humans are one of the few species where the female goes through a menopause which ends her reproductive years,” and added that the PNAS study found that “menopause appears to allow mothers to live longer and rear more offspring to adulthood.” The study also concluded that that “this unusual life history probably evolved in our species because offspring are so extremely dependent on their mother’s survival.”
Researchers note that natural selection does not necessarily favor maximal reproduction because reproduction “imposes fitness costs, reducing parental survival, and offspring quality.” According to Smith, “We found that increasing number of offspring and rates of reproduction were associated with reduced parental survivorship, and significantly more for mothers than fathers.”